This invention relates to shielded electrical cables generally, and particularly relates to a cable which has an improved metal foil shielding tape therein.
Cables of the type used to transmit high frequency signals (such as radio and television signals) usually have one or more inner conductors formed of copper or copper-clad aluminum, with the inner conductor or conductors being insulated by a dielectric material such as expanded or unexpanded polyethylene. The dielectric material, in turn, is surrounded by a metallic outer conductor. The metallic outer conductor, in addition to serving as an electrical conductor, also serves to shield the cable against leakage of radio frequency radiation. Cables of the type which employ a metallic foil as the outer conductor typically utilize a foil tape wrapped around the dielectric and bonded thereto by an adhesive. Typically, the adhesive is applied as a coating on one surface of the foil.
It is well recognized that any crimping, folding or bending of the metallic outer conductor layer is highly deleterious to the cable. Not only may such disruptions allow ingress of moisture into the cable, but they also significantly interfere with the R.F. propagation characteristics of the cable. Even a relatively small, microscopic disruption of these surfaces, as would occur from microbending, decreases the signal propagating properties of the cable.
Accordingly, one approach to this problem has been to make the metal foil layer relatively thick in order to provide the needed strength and integrity and to provide a thin coating of adhesive on the surface for bonding to the cable. However, the thickness, stiffness and cost of this type of shielding tape make it undesirable for many applications.
To overcome these limitations, the most popular approach has been to form the shielding tape of a multilayered laminated construction, comprised of one or more relatively thin metallic foil layers and additional adhesive and/or polymer film layers. By way of example, one such multilayered shielding tape disclosed in U.S. Pat. No. 3,721,597 consists of an inner thermoplastic film having foil layers adhesively bonded to the opposing surfaces thereof. While these types of shielding tapes offer some advantages over the earlier thick metal shielding tapes, their complexity of construction dictates that they are relatively expensive.
In the aforementioned shielding tapes an adhesive is used for bonding the metal foil to the cable dielectric, and shielding tapes of laminated construction also use adhesives to bond together the various layers. The adhesive most commonly used for these purposes has been a copolymer of ethylene and acrylic acid, since the ethylene acrylic acid (EAA) adhesive will effectively bond both to metal surfaces and to polyolefin surfaces.
Unfortunately, while EAA adhesives have excellent structural bonding properties, they have poor electrical properties. EAA adhesives contain a large number of polar carboxyl groups, which produce increased electrical dissipation in the cable at the high frequencies of the signals carried by the cable. In recognition of this problem, in commercial practice, the dissipation or loss contributed by the EAA adhesive is minimized by applying the minimum possible thickness of EAA adhesive to the foil. However, even at these minimal levels, the effect of the EAA adhesive is measurable.